CN106093568A - A kind of novel low-cost high-precision digital frequency meter - Google Patents

Abstract

The invention discloses a novel low-cost high-precision digital frequency meter, which relates to the technical field of digital frequency meters. It is a digital frequency meter based on FPGA. The front-end signal input conditioning adopts broadband amplifier AD8099 to amplify the weak signal. After being shaped by the high-speed comparator TLV3501, it becomes large and small. The waveform is suitable for the signal directly processed by FPGA; The timing changes transmit control, status, and data information. The CPU reads the count value from the FPGA, and determines the type of signal parameter to be measured according to the setting of the button selection module. After further calculation and processing, it is output to the OLED display. The beneficial effects of the present invention are: the frequency measurement method adopts the equal-precision measurement method, which has higher precision and wider measurement range than the direct frequency measurement method and the circumference measurement method, and can meet the requirements of the frequency range from 1 Hz to 100 MHz, and the precision is also higher than 0.01 %, the whole system has good real-time performance, high flexibility, low power consumption and low cost.

Description

A New Type of Low Cost and High Accuracy Digital Frequency Meter

technical field

The invention relates to the technical field of digital frequency meters, in particular to a novel low-cost high-precision digital frequency meter.

Background technique

The frequency meter is mainly composed of the following three parts: signal shaping, signal parameter measurement, single-chip microcomputer control and display. Among them, signal parameter measurement can be subdivided into modules such as frequency/period measurement, time interval measurement, and duty cycle measurement.

At present, signal amplification and shaping modules are commonly used in the prior art. One is to use discrete components to realize the pre-amplification and shaping function. Two triodes are used to amplify the signal and send it to the inverter for shaping. Through simulation, it is found that when the measured signal amplitude is small , the waveform quality obtained when the frequency is higher is poor, and when the frequency exceeds 50MHz, the output signal cannot meet the requirements at all. In addition, this solution requires a large number of discrete components, the system design is complex, debugging is difficult, especially the quantitative adjustment of the gain is difficult, and the stability is poor, and easy self-oscillation is another shortcoming of the circuit; the other uses AD811 proportional amplifier circuit The small signal is amplified, and the large signal is directly input to the comparator. Because the bandwidth product of AD811 is too small, the attenuation is serious when amplifying high-frequency signals.

At the same time, frequency/period measurement modules are also commonly used in the prior art. One is the time threshold measurement method, including direct frequency measurement and direct period measurement. When it is high, the error is large; the other is the combination method, which is more troublesome to implement, and the measurement accuracy is not easy to mention very high.

Finally, the existing MCU control and display modules have the following deficiencies: STC89C52 has high power consumption, limited I/O port resources, slow processing speed, and LCD 12864 has short lifespan and small screen size.

Contents of the invention

The object of the present invention is to aim at the defects and deficiencies of the prior art, and provide a new low-cost high-precision digital frequency meter with simple structure, reasonable design and convenient use. The frequency method and cycle measurement method have higher precision and wider measurement range, which can meet the requirements of the frequency range from 1Hz to 100MHz, and the accuracy is higher than 0.01%. The whole system has good real-time performance, high flexibility and low power consumption.

In order to solve the existing problems in the background technology, the technical solution adopted by the present invention is: it includes a signal shaping module, a signal parameter measurement module, a single-chip microcomputer control and a display module; the signal parameter measurement module can be subdivided into frequency/period measurement , Time interval measurement, duty ratio measurement module.

Described signal shaping module adopts AD8099 proportional amplifying circuit, hysteresis comparator, input and output impedance matching, the output signal clutter of this amplifying circuit is more, the output signal waveform of amplifier is thicker, the threshold voltage of suitably adjusting hysteresis comparator can be Effectively suppress the impact of clutter on the subsequent comparator, so that when the input signal is a square wave, the signal frequency and effective value range are relatively wide, and when the input signal is a sine wave, it can also meet the requirements.

The frequency/period measurement module adopts an equal-precision measurement method, and two sets of counters count simultaneously within the same time threshold, and start counting when the gate signal is valid and the rising edge of the signal to be measured arrives, and when the falling edge of the gate threshold arrives, only The two sets of counters stop counting at the end of a cycle of the signal to be measured.

The single-chip microcomputer control and display module adopts STM32 single-chip microcomputer, 32-bit Cortex-M3CPU, and the display module adopts OLED.

The front-end signal input conditioning circuit adopts broadband amplifier AD8099 to amplify the weak signal, and after being shaped by the high-speed comparator TLV3501, it becomes a signal whose size and waveform are suitable for direct processing by FPGA; the control status and data are transmitted between the STM32 microcontroller and FPGA according to the timing change Information, the CPU reads the count value from the FPGA, determines the type of signal parameters to be measured according to the setting of the button selection module, including frequency/period, time interval, duty cycle, etc., and then outputs to the OLED display after further calculation and processing.

After adopting the above-mentioned structure, the beneficial effects of the present invention are: the method of measuring the frequency adopts the equal precision measurement method, which has higher precision and wider measurement range than the direct frequency measurement method and the cycle measurement method, and can meet the requirements of the frequency range from 1 Hz to 100 MHz. The accuracy is also higher than 0.01%, and the whole system has good real-time performance, high flexibility and low power consumption.

Description of drawings

Fig. 1 is a system overall block diagram of the present invention;

Fig. 2 is a signal shaping block diagram of the present invention;

Fig. 3 is a signal measurement block diagram of the present invention;

Fig. 4 is a block diagram of the control and display of the single chip microcomputer of the present invention.

detailed description

Below in conjunction with accompanying drawing, the present invention will be further described.

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, and are not intended to limit the present invention.

As shown in Figures 1-4, this embodiment adopts the following technical scheme: it includes a signal shaping module, a signal parameter measurement module, a single-chip microcomputer control and a display module; the signal parameter measurement module can be subdivided into frequency/period measurement, time interval measurement, duty cycle measurement module.

Described signal shaping module adopts AD8099 proportional amplifying circuit, hysteresis comparator, input and output impedance matching, the output signal clutter of this amplifying circuit is more, the output signal waveform of amplifier is thicker, the threshold voltage of suitably adjusting hysteresis comparator can be Effectively suppress the impact of clutter on the subsequent comparator, so that when the input signal is a square wave, the signal frequency and effective value range are relatively wide, and when the input signal is a sine wave, it can also meet the requirements.

The frequency/period measurement module adopts an equal-precision measurement method, and two sets of counters count simultaneously within the same time threshold, and start counting when the gate signal is valid and the rising edge of the signal to be measured arrives, and when the falling edge of the gate threshold arrives, only The two sets of counters stop counting at the end of a cycle of the signal to be measured. This overcomes the problem that the pulse period of the signal to be measured is incomplete. The error is only generated by the standard frequency signal and has nothing to do with the frequency of the signal to be measured. The maximum error is plus or minus one standard frequency period, and the standard signal is 10MHz. The accuracy reaches 10 ^-7 , which can fully meet the required accuracy.

The single-chip microcomputer control and display module adopts STM32 single-chip microcomputer, 32-bit Cortex-M3CPU, which has low power consumption mode, faster processing speed, and more I/O ports, which can facilitate the communication with FPGA and peripheral hardware; the display module adopts OLED is superior to LED in thickness and weight, and has a wide range of viewing angles, high luminous efficiency, and low energy consumption. At the same time, compared with OLED, LED still has disadvantages such as insufficient color purity, short life, and screen size. Smaller etc.

Signal frequency/period measurement: Regardless of whether the measured signal is a sine wave or a square wave, it will become a square wave that can be directly processed by the FPGA after the shaping circuit. Using the equal-precision measurement method mentioned in the program demonstration, the required 1 For the frequency/period of ~100MHz signal, choose 10MHz square wave as the standard signal, which can greatly improve the measurement accuracy.

Same-frequency square wave time interval measurement: After processing the same-frequency square-wave signal entering the FPGA, when its frequency is low, perform XOR operation on two square waves with the same frequency and time interval to obtain a rectangular wave. In order to improve the measurement accuracy, you can compare the duration of the high level and low level of the rectangular wave within a cycle, and the result obtained by timing the long duration shall prevail. The time interval of the same frequency signal is the corresponding time of the count value or the period and the corresponding time Poor; but if the signal frequency is very high, continue to use this scheme, especially when the rectangular wave duty cycle is close to 50%, because the counting number of the reference signal in the interval time is very small, and the ±1 error generated by the counting is objective, so To achieve the required accuracy, the frequency of the standard signal has to be continuously increased.

One of the requirements for a square wave as a reference signal is stability and a regular waveform, and at the same time, the higher the frequency, the better. Since the maximum frequency of the square wave generated by the FPGA is 400MHz, considering the situation where the measurement error is the largest, that is, when the duty cycle of the rectangular wave is 50%, if you want to achieve an accuracy of 1%, the number of reference signals in one cycle of the rectangular wave It should not be less than 200. At this time, the maximum frequency of the rectangular wave is 2MHz. Correspondingly, the maximum frequency of the measured square wave with the same frequency is 1MHz. Accuracy requirements can be met.

When the frequency of the signal to be measured is greater than 1MHz, in order to ensure the accuracy of the measurement, the frequency of the rectangular wave obtained by XOR can be divided by 1000, and the part of the high level in one cycle obtained after frequency division can be used as the gating signal, and two counters are used to gate When the control signal is valid, the high level and low level of the rectangular wave contained in it are counted corresponding to the number of the reference square wave, and the time is calculated based on the longer duration. Since there is an error of ±1 in the number of counts in each rectangular wave cycle, in extreme cases, the error of the total number of counts is ±1000. At this time, the measurement accuracy is the same as that of no frequency division, but when this happens The probability is extremely small, and when the ±1 error generated by the counting of each cycle of the rectangular wave can partially cancel each other due to the opposite sign, the measurement accuracy can be improved accordingly. For example, if the total count error is 200, then the frequency division The measurement accuracy is 5 times that of the case of no frequency division. It is worth noting that the probability of an error of 200 is much greater than that of an error of 1000.

Measures to improve the sensitivity of the instrument: use the equal precision measurement method, and the ±1 error of the original number of measured signals is converted into the ±1 error of the reference signal when counting; coupling capacitors are added to both ends of the op amp of the preamplifier shaping circuit, and the entire system Eliminate noise interference caused by factors such as power supply and environment as much as possible; the circuit uses PCB boards, and chips are mostly packaged with SMDs, which reduces the interference introduced during wiring; select comparators with excellent performance, operational amplifier chips, etc., to reduce artificial The error caused by factors; when introducing the measured signal, use coaxial cable and BNC connector to reduce signal attenuation.

The FPGA-based digital frequency meter of the present invention utilizes the Verilog hardware description language to design and realize the internal function modules of the frequency meter, and completes the logic simulation on the software platform Quartus. The front-end signal input conditioning adopts the broadband amplifier AD8099 to amplify the weak signal, and after being shaped by the high-speed comparator TLV3501, it becomes a signal whose size and waveform are suitable for direct processing by the FPGA; the control status and data information are transmitted between the STM32 microcontroller and the FPGA according to the timing change , the CPU reads the count value from the FPGA, and according to the setting of the key selection module, determines the type of signal parameters to be measured, including frequency/period, time interval, duty cycle, etc., and then outputs to the OLED display after further calculation and processing.

The method for measuring the frequency in this specific embodiment adopts the equal-precision measurement method, which has higher precision and wider measurement range than the direct frequency measurement method and the circumference measurement method, and can meet the requirements of the frequency range from 1 Hz to 100 MHz, and the accuracy is also higher than 0.01%. The system has good real-time performance, high flexibility, low power consumption and low cost.

The above is only used to illustrate the technical solution of the present invention and not to limit it. Other modifications or equivalent replacements made by those skilled in the art to the technical solution of the present invention should be considered as long as they do not depart from the spirit and scope of the technical solution of the present invention. fall within the scope of the claims of the present invention.

Claims (2)

1. A novel low-cost high-precision digital frequency meter is characterized in that it comprises a signal shaping module, a signal parameter measurement module, a single-chip microcomputer control and a display module; the signal parameter measurement module can be further subdivided into frequency/period measurement, time Interval measurement, duty ratio measurement module; the signal shaping module adopts AD8099 proportional amplifier circuit, hysteresis comparator, input and output impedance matching; Simultaneous counting within the threshold, start counting when the gate signal is valid and the rising edge of the signal to be measured arrives, and when the falling edge of the gate threshold arrives, the two groups of counters stop counting only when one cycle of the signal to be measured ends; the single-chip control And the display module adopts STM32 single-chip microcomputer, 32-bit Cortex-M3CPU, and the display module adopts OLED. 2. A new low-cost high-precision digital frequency meter according to claim 1, characterized in that the front-end signal input conditioning adopts a broadband amplifier AD8099 to amplify the weak signal, and after being shaped by the high-speed comparator TLV3501, it becomes large and small. Signals suitable for direct processing by FPGA; STM32 MCU and FPGA transmit control status and data information according to timing changes, CPU reads the count value from FPGA, and determines the type of signal parameters to be measured, including frequency, according to the setting of the button selection module /period, time interval, duty cycle, etc., and then output to OLED display after further calculation and processing.